Diffusers for centrifugal compressors are broadly classified into vaneless diffusers and vaned diffusers. Among these, the vaned diffuser changes the direction of flow by vanes, and also decelerates the flow by the vanes, and therefore generally the efficiency in the vicinity of a design flow rate is higher as compared with the vaneless diffusers. However, the efficiency is lowered at the large flow rate and the small flow rate because of an increased loss by the vanes and the stall of the flow by the vanes, so that the range of the operation is narrow.
Therefore, there has been proposed a diffuser with a small chord-pitch ratio (hereinafter referred to as "low solidity vaned diffuser) disclosed in Japanese Patent Unexamined Publication No. 53-119411, in which the operation range is not so narrowed even with the use of a vaned diffuser, and the efficiency can be enhanced to a certain degree.
Japanese Utility Model Unexamined Publication No. 56-97598 discloses an example in which a channel height is abruptly narrowed at a vaneless portion provided downstream of a low solidity vaned diffuser, and the channel length of the vaneless portion is shortened so as to reduce a friction loss. Further, Japanese Patent Unexamined Publication No. 1-125599 discloses an example in which in a compressor stage of a relatively low specific speed, a channel height of a vaned portion of a vaned diffuser is decreased progressively in a downstream direction so as to enhance the efficiency by reducing a friction loss.